The present invention relates to a fluid thrust bearing apparatus wherein a rotatable element is prevented from moving axially out of a close-fitting cavity by cohesive and adhesive forces of a liquid disposed between the rotatable element and the cavity walls and negative (vacuum) pressure created when the rotatable element is axially stressed.
1. Field of the Invention
The present invention relates to the field of thrust bearings used to prevent axial movement of a shaft disposed in a cavity.
2. Description of Related Art
Most indicating instruments (e.g., ammeters, Bourdon tube pressure gauges, etc.) include a shaft, which rotates between radial and thrust bearings. The radial and thrust bearings prevent radial and axial movement of the shaft. The shaft is connected to a pointer that is juxtaposed against a dial, and the amount of shaft rotation correlates to the value of the parameter being measured (e.g., electrical current, pressure, etc.). In this way, the pointer indicates on the dial the value of the parameter.
Most indicating instruments are inexpensive. It is therefore critical that the number of parts are kept to a minimum. Further, it is preferred that manufacturing procedures are simplified to minimize labor costs. The elimination of even a single thrust bearing can provide a significant advantage to indicating instrument manufacturers.
The mechanism employed to create shaft rotation in a particular indicating instrument depends on the parameter being measured. For example, moving-magnet indicating instruments are well known for measuring electrical current. A movingmagnet indicating instrument of the cross-coil type is described in U.S. Pat. No. 5,095,266, which was issued to Yukio Ohike, et al. on Mar. 10, 1992. That patent discloses a pair of mutually perpendicular coils surrounding a magnetized rotor. The rotor is disposed in a cavity and connected to a shaft and associated pointer. Currents that vary with the momentary magnitude of the parameter being measured flow through the coils creating a correspondingly varying magnetic field. The magnetic field created by the current causes proportional rotation by the rotor and, in turn, the shaft and pointer.
By contrast, a "Bourdon tube" apparatus can be used to measure pressure. A Bourdon tube is a curved tube that is closed on one end and connected at its open end to the pressurized cavity to be analyzed. Pressure in the cavity deforms the tube, and the deformation of the tube is mechanically translated into rotation of a gear shaft. The gear shaft is coupled to a pointer, which indicates the measured pressure on a dial.
Most indicating instruments are characterized by low inertia and low friction, thus necessitating damping means. Typically, damping is provided by fluid, such as silicone oil, disposed between the rotating shaft or rotor and the walls of the cavity in which it sits. Unfortunately, most suitable damping fluids tend to migrate through the clearance between the pointer shaft and the instrument housing. This can cause the damping fluid to escape the instrument entirely or migrate into areas of the instrument where it is not desired, such as the pointer, the dial face, or the lens of the instrument. Applicant's U.S. patent application Ser. No. 09/146,741, filed Sep. 3, 1998, which is herein incorporated by reference, describes preferred "migration barriers" that can be used to prevent unwanted migration of damping fluid.
Another common characteristic of electrical and mechanical indicating instruments is that the shaft connected to the pointer must be prevented from moving axially. In most indicating instruments, axial movement is prevented by two physical thrust bearings--mechanical barriers to axial movement of the shaft in either direction. Alternatively, some prior art instruments discourage axial movement magnetically. For example, U.S. Pat. No. 4,710,706, which issued to Robert Krupa on Dec. 1, 1987; U.S. Pat. No. 3,013,210, which issued to Peter Wargo on Dec. 12, 1961; and U.S. Pat. No. 2,867,768, which issued to Austin E. Fibrance, et al., on Jan. 19, 1953, describe the use of a holding magnet to hold a magnetized shaft or rotor against a single thrust bearing, thereby preventing axial movement.
The holding magnets disclosed in these patents also bias control the magnetized shaft and return the pointer to the zero position after electrical input to the measuring device is cut off. This combination of a magnetic thrust bearing and a return-to-zero mechanism reduces the number of necessary parts for the instrument. However, a holding magnet is an inappropriate return-to-zero mechanism for some indicating instruments. For example, the shaft used in a mechanical indicating instrument, such as a Bourdon tube pressure gauge, is usually not magnetized and cannot be controlled with a magnet. Moreover, a variety of other return-to-zero mechanisms (e.g., hairsprings) are available that may be advantageous in certain applications.
What is needed is an apparatus for preventing axial movement of a rotatable element that reduces the number of necessary parts, e.g., by eliminating the need for a holding magnet or one of the two separate mechanical thrust bearings employed in the prior art.
What is needed is a thrust bearing that damps rotational movement of a rotatable element while also preventing axial movement.
What is needed is an apparatus for preventing axial movement of a rotatable element that can be employed in any indicating instrument, including electrical and mechanical indicating instruments.
What is needed is an apparatus for preventing axial movement of a rotatable element that can be employed in an indicating instrument with any return-to-zero mechanism.